In order to manufacture an optical disc such as a video disc, read-only digital audio disc and writable optical disc having prepits and pregrooves, a master disc is cut with a laser beam to record information thereon. In general, the master disc comprises a polished glass substrate on which a photoresist is applied. The photoresist is irradiated by a laser beam which is modulated in accordance with a signal to be recorded. The irradiated portions of the photoresist are thus developed and removed.
In order to record signals on the master disc, the surface of the disc is processed by cutting with a gas laser beam emitted from a gas laser. Namely, the gas laser beam is turned on and off by an optical modulator in accordance with the signal to be recorded. The emitted laser beam is transmitted through a mobile optical recording system and an objective, and applied to the photoresist on the glass substrate which is rotated by a spindle motor.
A conventional argon gas laser is a common gas laser as a cutting device for the disc.
Referring to FIG. 8, the gas laser has a gas-discharge tube 1 filled with argon gas, and a discharge device is provided in the tube. The tube 1 further has a first and second apertures 2 and 3 at both ends thereof. The gas laser is provided with an output mirror 4 and a rear mirror 5 which are disposed at the front and rear of the discharge tube 1, respectively, thereby forming an optical resonator. The output mirror 4, provided to confront the first aperture 2, comprises a partial reflecting mirror. The rear mirror 5, disposed at the end of a beam path, comprises a total reflecting mirror. A wavelength selecting prism 6 is disposed between the second aperture 3 of the discharge tube 1 and the rear mirror 5.
In operation, when excited, the discharge tube 1 oscillates a laser beam. The generated laser beam is resonated by the output mirror 4 and the rear mirror 5. Although the argon gas laser oscillates a laser beam having various wavelengths, the wavelength selecting prism 6 selects a spectrum at a predetermined range. Namely, as shown in FIG. 9, a spectrum of wavelengths surrounded by a dotted line is selected.
The wavelengths of the laser for the cutting device has relatively low gain. Therefore, it is necessary to increase the gain range. To this end, the length of the resonator, that is the distance between the output mirror 4 and the rear mirror 5, is set large. As a result, the interval between the longitudinal modes, which depends on the resonator length, is reduced. Moreover, since the spectrum has a range of several megahertz, the laser beam has a plurality of longitudinal modes. Namely, the gas laser beam oscillates in multiple longitudinal modes. When the transverse mode of the laser is TEM.sub.00, in a frequency range between two and fifteen megahertz, no noise is generated. Hence the laser beam is appropriate for cutting the master disc.
In an optical video disc from which a digital sound is also reproduced, a frequency-modulated (FM) analog audio signal and a digital modulation audio signal, more particularly, eight to fourteen modulation (EFM) audio signal are multiplexed in a band lower than the FM video signal. The EFM audio signal occupies the band ranging from several ten kilohertz to about two megahertz. In order to restrain the interference of the EFM audio signal on the FM video signal, the level of the EFM audio signal is multiplexed at a level lower than that of the FM video signal by twenty decibels. Hence when recording a signal having a frequency lower than two megahertz, there may occur a problem that low frequency noise is generated in the cutting device.